1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle for maintaining wheel slippage during vehicular braking state in the vicinity of a predetermined optimum level. More specifically, the invention relates to an anti-skid brake control system which can precisely derive a vehicle body representative data as one of the essential parameters for precisely performing anti-skid brake control.
2. Description of the Background Art
One of the typical conventional anti-skid brake control systems has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 57-11149. In the disclosed technologies, the anti-skid brake control system derives a vehicle body speed representative data on the basis of a longitudinal acceleration exerted on the vehicle body. In the practical operation for deriving the vehicle body speed representative data, a wheel speed upon initiation of vehicular braking operation is latched as an initial value of the vehicle body speed representative data. The longitudinal acceleration exerted on the vehicle body is monitored by a longitudinal acceleration sensor which produces a longitudinal acceleration indicative signal. The longitudinal acceleration indicative signal is integrated. The integrated value is subtracted from the initially latched wheel speed as the initial value of the vehicle body speed representative data in order to derive an instantaneous value of the vehicle body speed representative data.
On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 56-79043 discloses another system of the anti-skid brake control system. The disclosed system detects high possibility of wheel locking. The system is responsive to the vehicular braking condition which can cause wheel locking to latch an instantaneous wheel speed as the initial value of the vehicle body speed representative data. For a given period from detection of possibility of wheel locking, a fixed value, e.g. -1.2 G, of a wheel deceleration gradient data. The wheel deceleration gradient data is integrated over the given period. The system subtracts the instantaneous integrated value from the initially locked wheel speed data for deriving the instantaneous vehicle body speed representative data.
In the former case, error contained in the longitudinal acceleration indicative signal due to secular variation which may cause variation of gain or drift of the longitudinal acceleration sensors per se. Influence of the error on the longitudinal acceleration indicative signal makes the signal value to represent the smaller magnitude of longitudinal acceleration than an actual value. In such case, the vehicle body speed representative data projected based on the integrated value of the error containing longitudinal acceleration indicative signal, tends to become higher than the actual vehicle body speed. Therefore, a wheel slippage derived based on such error containing projected vehicle body speed representative data can become greater than an actual value to cause decreasing of the braking pressure in a wheel cylinder at earlier timing than that actually required. This clearly degrade vehicular braking performance.
On the other hand, in the latter case, since the fixed value of the wheel deceleration gradient indicative value is set irrespective of the friction coefficient on the road surface. Therefore, braking performance can be significantly varied depending upon the friction level of the road surface. Furthermore, in case of four wheel-drive type vehicle, error between the projected vehicle body speed representative data and the actual vehicle body speed due to substantially great inertia.